In Disclosure Document No. 367510, submitted to the PTO on 27 Dec. 27, 1994, there appears much of the material in this specification.
Present dental practices tend toward the replacement of lost teeth with cylindrical or plate metal alloy implants embedded in the bone of the mandible or maxilla to support the artificial tooth restoration. If extensive replacement of several teeth is needed, several implants, alone or in conjunction with existing teeth prepared as abutments, are used to anchor the replacement prosthetic teeth. As the number and complexity of support abutments are increased, the difficulty in aligning and fastening the prosthetic restoration increases. The use of several mechanically connected parts at each post or plate implant site, adds to the possibility of misalignment or biomechanical failure. This invention describes a method and apparatus to provide a simpler mechanism and more direct technique for securing a permanently cemented, yet retrievable prosthodontic appliance while still offering a durable mechanical support. The last two decades have led to a revolution in implant prosthodontics. Titanium alloy implant cylinders or plates are intimately installed in holes or slots drilled in the underlying bone. It is the practice to allow several months to pass while the underlying bone bonds to the surface of the implant. For this reason, implants are provided with at least one threaded hole on the crestal surface or edge. These holes are temporarily capped with a healing screw to prevent the downgrowth of soft tissue and bone into the internal threads. The soft tissue is sutured over the implant until the intimate metal-bone bond is effected.
At the next surgical encounter, the soft tissue is resected and the healing screw is replaced with a metal alloy perimucosal extension of selectable height and emergence profile and the soft tissue is sutured around the base of this extension. This extension is usually bolted in place and prevented from rotating by means of locating pins and holes or internal and external matching hexagonal (or other regular polygon shaped) projections. These perimucosal extensions form the support for artificial abutments used to support the final prosthetic restoration. The final prosthodontic restoration requires a close mechanical mating between the abutments and the matching internal aspect or underside of the prosthesis. These closely matched parts often consist of telescoped, tapered cylindrical surfaces requiring a tight, non-binding, "passive" fit. This places inordinate requirements on the precision and technical skills of the dentist and the laboratory technician. Parallel alignment of the axes of each abutment to prevent binding of tapered fits cannot be easily guaranteed. The present invention, relying on a conformable, cemented boundary, circumvents these objections.
Much of the current discussion in the field of dental implantology centers around the durability and maintainability of the various methods of attaching the final restoration to the underlying abutments. Bolting with threaded fasteners through the occlusal surface of the restoration and back filling with composite materials complicate the cosmetics and the retrievability of the prosthesis. Bolting through the non cosmetic, lingual side of the prosthesis has the additional requirement for a greater thickness of metal to provide mechanical support, thus reducing room for the tongue and potentially affecting speech, and the periodontal health of the abutment.
Excessive inline or rocking pressure transmitted to an individual implant from the overlying restoration may lead to frank implant failure. Failures may occur from the loosening of a screw caused by thread walking or the backing out of a screw by micro-movements. The shifting of an abutment from repetitive stresses exceeding the elastic limits between the screw thread and the internal thread of the implanted post or plate may cause the flexure or excessive loading of a single implant. Long term changes in the underlying bone structure in response to uneven stresses may lead to the loss of an individual dental implant. For each additional mechanically attached connection, alignment errors accumulate and reduce the likelihood of a good non-binding, stress free "passive" fit.
The present invention acts to equitably distribute the loading forces with a retrievable dental cement between the matching faces of the abutments and the internal aspect of the final restoration. Each abutment is made with at least one step or shelf on the lingual face to act as a bearing surface for a removal instrument. The final prosthesis is equipped with a flat-topped window ledge on the lingual side. There is a matching shelf on the implant abutment, with enough space between the surface of the shelf and the flat top of the window for the introduction of a wedge-tipped extraction instrument. This instrument is used to apply a prying force between each abutment and the mating ledge in the underside of the final restoration. The prying instrument applies an even opposing force between the overstructure and the abutment eliminating the potential damage to both structures. Prior methods of removing cemented restorations involved hammering movements under much less control. The method and apparatus of this invention in combination with an appropriate dental cement, yields a predictable technique for securing, yet retrieving the final restoration.